The present invention, in some embodiments thereof, relates to image processing and, more particularly, but not exclusively, to a method and a system for reducing chromatic aberration.
Imaging systems employ optical elements such as lenses or lens assemblies that focus incoming light onto a light sensor element. Lenses and lens assemblies oftentimes have aberrations which can degrade the quality of images captured by such the imaging system. One type of aberration is chromatic aberration which results from the fact that different wavelengths or colors of light are refracted by different amounts by the lens or lens assembly.
Chromatic aberration appears when a lens is transmitting polychromatic light. Since the index of refraction of optical glass is wavelength dependent, the red, green and blue components bend differently at an optical interface in the lens. This leads to longitudinal (axial) and/or transverse chromatic aberration effects. When a lens fails to focus various colors sharply in the same plane, the lens is said to exhibit longitudinal chromatic aberration (LCA).
In longitudinal chromatic aberration, the three components are brought to focus on different planes in the image space, which gives a color blurring effect. Thus, longitudinal chromatic aberration arises due to the focal length varying with wavelength.
In transverse chromatic aberration, color components from a single point are brought to focus to different points on the same image plane, resulting in a lateral shift of the image. This has the effect of magnifying the different wavelength differently and can be visually seen as color fringing. Thus, transverse chromatic aberration can be seen as an effect due to magnification varying with wavelength.
Chromatic aberrations are channel dependent in the sense that each color channel, e.g., red, green and blue channels, provides a different amount of the aberration artifact in the image plane. In order to characterize these aberrations, the ability of a lens to transfer information from the object to an image plane is represented as a modulation transfer function (MTF). A lens MTF is a measure of how well the original frequency-dependent contrast of the object is transferred to the image.
Because the distortion introduced by aberrations into an optical system significantly degrades the quality of the images on the image plane of such system, there are advantages to the reduction of those aberrations. A great deal of the complexity of modern lenses is due to efforts on the part of optical designers to reduce optical aberrations. An alternative approach is to compensate for these aberrations in the reproduction process so that final images with reduced aberrations may be obtained.
U.S. Application Publication No. 20090263018 discloses an image processing apparatus that reduces color blur included in an image which is constituted by plural color planes. The apparatus reduces a component of chromatic aberration of magnification which is included in the image, and a component of longitudinal chromatic aberration which is included in the image after the component of chromatic aberration of magnification has been reduced. The component of longitudinal chromatic aberration is calculated based on a distance from a brightness saturation area in the image.
U.S. Pat. No. 8,358,835 discloses a technique for detecting and correcting a chromatic aberration. A chromatic aberration is detected by (i) dividing an edge portion of the image into static areas where a color varies below a reference value and a dynamic area where a color varies above the reference value, (ii) calculating a intensity difference value between a reference color and a comparative color for each of the static areas and the dynamic area, and (iii) determining a chromatic aberration region when an intensity difference value exceeds a range of intensity difference values in the static areas among pixels in the dynamic area. The chromatic aberration is corrected by correcting the intensity difference value for the pixels in the chromatic aberration region using a correction value in the intensity difference value range in the static area.